This application claims, under 35 USC 119, priority of Japanese Application No. 2002-191522 filed on Jun. 28, 2002.
The disclosure of Japanese Patent Application No. 2002-191522 filed on Jun. 28, 2002, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an oil pressure control device for an automatic transmission installed in a motor vehicle and, more particularly, to an oil pressure control device for an automatic transmission including a control valve that supplies engaging pressure to a friction engagement element, such as a clutch or the like, wherein the control valve is susceptible to sticking.
2. Description of the Related Art
Conventional automatic transmission oil pressure control devices (hereinafter, also referred to as xe2x80x9coil pressure control devicesxe2x80x9d) have a plurality of shift valves, control valves, etc., that are interconnected to form a circuit, and which cooperate to change the shift speed in accordance with operation of a shift lever. Some of such oil pressure control devices are designed to supply a predetermined range pressure, such as a D (drive) range pressure or the like, to a hydraulic servo of a so-called vehicle launch clutch used at the time of starting the vehicle in motion, such as a clutch C-1, via the control valve, and to discharge the engaging pressure of the hydraulic servo via the control valve at the time of, for example, D-N control, i.e., in a shift from the D range to the N (neutral) range.
However, there is a possibility that small foreign substances in the automatic transmission fluid (ATF) (hereinafter referred to as xe2x80x9coilxe2x80x9d), which is drained from the hydraulic servo by the aforementioned D-N control, will cause sticking of the control valve and, more particularly, intermediate sticking in which the spool of the control valve becomes stopped at an intermediate position. If such intermediate sticking occurs in, for example, an automatic speed change mechanism 10 as illustrated in FIG. 1, it becomes difficult to fully release the clutch C-1; furthermore, there is the danger that at the time of a N-R shift, i.e., a shift from the N range to the R range, the clutch C-3 which should engage in R range (see FIG. 2) might become tied up with the clutch C-1 in a not fully released state, thus malfunctioning and accelerating abrasion of the clutch friction plates.
In the conventional oil pressure control devices as described above, the speed of oil drainage from the hydraulic servos is dependent on oil viscosity which, in turn, varies with ambient temperature. That is, at a high temperature the oil viscosity is lower than at a low temperature. Therefore, if a D-N shift is performed at high temperature, a clutch, such as the clutch C-1 or the like, may rapidly release, causing what is generally termed a torque escape shock. At low temperature, the oil viscosity increases, and therefore the draining speed decreases, so that problems such as delayed clutch release, or the like may occur.
With the aforementioned conventional oil pressure control device, in D range a line pressure is supplied as a range pressure to the control valve via a manual valve, and in N range the control valve is connected to a drain port at the side of the manual valve. Therefore, the oil drained from the hydraulic servo during D-N control is rapidly drained from the control valve via the manual valve, creating the possibility of shock caused by rapid torque loss at a friction engagement element.
Accordingly, it is an object of the present invention to provide an oil pressure control device for an automatic transmission which is designed so that at the time of release of a friction engagement element, such as the C-1 clutch or the like, the engaging pressure can be appropriately drained from the hydraulic servo while sticking of a valve within the pressure adjusting unit, such as a control valve or the like, is avoided, whereby the above-noted problems are solved.
It is another object of the present invention to provide an oil pressure control device for an automatic transmission which is designed so as to achieve the aforementioned object, and to prevent the discomfort of shocks caused by rapid oil drainage due to a change in oil viscosity or the like, and to mitigate the problem of the drainage speed decreasing with increases in oil viscosity.
According to one aspect of the present invention, a bypass drain oil passage is provided for draining an engaging pressure while bypassing the pressure adjusting unit during release of a friction engagement element. Therefore, it is possible to achieve good drainage of the engaging pressure from the hydraulic servo even in the case of valve sticking or other undesired event within the pressure adjusting unit. The input port and the output port may be connected during drainage of the hydraulic servo, depending on the operation of the pressure adjusting unit. In such a case, the one-way valve is able to reliably prevent the drainage of the engaging pressure from the hydraulic servo toward the manual valve via the connected output port and input port.
The bypass drain oil passage may be in the form of first and second drain oil passages that form different paths between the hydraulic servo and the manual valve. Therefore, the oil pressure drainage can be appropriately changed between a relative slow drainage using one of the first and second drain oil passages, and a relatively fast drainage using both drain oil passages. The setting of a slow drainage, for example using only the first drain oil passage, will offset the effect of a decrease in the oil viscosity or the like at high temperature, and therefore will prevent shock which would otherwise be caused by a rapid torque loss at the friction engagement element. Conversely, a high drainage rate using both the first and second drain oil passages will increase the drainage speed, will tend to offset the effect of an increase oil viscosity or the like at low temperature, and therefore will allow prompt release of the friction engagement element.
The second drain oil passage may be provided with a changeover valve that can be switched between positions respectively connecting and disconnecting the second drain oil passage and the first drain oil passage. Therefore, by controlling the changeover valve, the drainage of the engaging pressure from the hydraulic servo can be appropriately and easily changed in accordance with the ambient temperature and the like.
The present invention avoids, to the extent possible, the occurrence of the valve sticking and, more particularly, intermediate sticking, which is likely to occur within the clutch control valve because of its precision structure needed for a high level of responsiveness to the control pressure.
According to the present invention, if valve sticking or the like occurs in the pressure adjusting unit at the time of engagement or disengagement of the vehicle launch clutch, it is possible to reliably prevent, for example during a shift from the first forward speed to the reverse speed, the launch clutch from functioning simultaneously with a friction engagement element, such as a clutch engaged in reverse, which would accelerate abrasion of the clutch plates.